Multiplex radio system of the carrier frequency shift type



April 9, 194:6 P.` BERNsTElN T 2,397,913

V MULTIPLEX RADIO SYSTEM OF THE CARRIER FREQUENCY SHIFT TYPE Filed July 19 Patented Apr. 9, 1946 MULTIPLEX RADIO SYSTEM OF THE CARRIER FREQUENCY SHIFT TYPE Philip Bernstein, Chicago, Ill., assignor to Press Wireless, Inc., Chicago, Ill., a corporation of Delaware Application July 19, 1944, Serial No. 545,629

Claims.

This invention relates to multiplex radio systems and more especially to such systems using carrier frequency shift.

One of the principal objects is to extend the usefulness of an existing radio service transmitter by fully employing the frequency tolerance limits of the normal assigned service frequency of the transmitter.

Another principal object is to provide a method of operating an existing radio service transmitter by the carrier frequency shift method, whereby it is possible to key the transmitter to twoor more channels simultaneously, without the usual loss of power, and without the phase distortion and troublesome side bands encountered with well-known modulated-tone multiplex methods.

A further object is to provide an improved shift-control arrangement for multiplex radio transmitters of the carrier frequency shift type.

A feature of the invention relates to an electronic tube switching arrangement for controlling the effectiveness of a plurality of carrier frequency shift channels.

A further feature relates to the novel organization, arrangement and relative interconnection of parts which cooperate to provide an improved and highly exible multiplex radio service system.v

Other features and advantages not specically enumerated will be apparent after .a consideration of the following detailed descriptions and the appended claims.

In the drawing which represents one preferred embodiment,

Fig. 1 is a schematic Wiring diagram of a transmitter and receiver system embodying the inventive features.

Fig. 2 is an explanatory wave diagram applicable to Fig. 1.

Referring to Fig. 1, the transmitter portion of the system comprises any well-known type of radio service transmitter I, which generates a carrier having an assigned service frequency with certain fixed frequency tolerances on either side of the assigned frequency. Associated with the transmitter is an arrangement 2, for shifting the carrier frequency between fixed limits under control of signals to be transmitted. Transmitter I has also associated therewith amplitude modulation equipment 3, whereby another signal can be simultaneously transmitted 'by amplitude modulation of the frequency shifted carrier. For a detailed description of the arrangements 2 and 3, reference may be had to application Serial No. 498,278, filed August 12, 1943.

In accordance with the present invention, the

I2 and the vacuum-tube amplifier I3.

carrier shiftdevice 2 is arranged to bewkeyed by two separate message channels 4 and 5. For this purpose, the channel 4` is arranged to shift the carrier frequency of the transmitter I a pre- Y determined amount above its normal assigned frequency; while channel 5 is arranged to shift the carrier frequency a predetermined amount below its normal assigned frequency. Each of the channels 4 and 5 is controlled at a predetermined frequency rate by means of a common master oscillator 6, whose frequency determines the rate at which the transmitter I is shifted'in frequency. For example, the normal assigned frequency of transmitter I may be 10,000 k. c., and the oscillator 6 may operate at a frequency of 0.9 k. c.

The control oscillator 6 is connected to the primary of the audio frequency transformer 1 which has two similar secondaries 8 and 9. The secondary 8 has its opposite ends connected respectively to the control grids I 0, II, of the gas triode Likewise, the secondary 9 has its ends connected to the grids I4, I5, of gas triode I6 and vacuum-tube amplifier I1. Tubes I2 and I6 may for example be of the 884 'I'hyratron type, while tubes I3 and I'I may be of the beam power type such as a GLB. The secondaries 8 and 9 are connected to the respective grids through condensers I8, I9, 20 and 2|. Each pair of tubes I2, I3, and I6, I1, acts respectively on the half waves from source 6 to convert `them into square-topped pulses as indicated in Fig. 2. The transformer secondaries are so phased that they excite the grids I I and I4 in like phase; and they excite the grids I 0 and I5 also in like phase but degrees displaced with respect to the excitation of grids I I and I4. Consequently,v when a positive pulse is impressed on grids Il and I4, a negative pulse is simultaneously impressed on grids I0 and I5. kWith a positive pulse on grid II, the plate current of tube I3 will rise and cause an increase in voltage drop in the plate load resistor 22. This also causes a decrease in the plate voltage applied to the gas triode I2 to suchy an extent that the plate current thereof is extinguished. This abrupt break in the conductivity of tube I2 is further accelerated by the fact that :at the same time the negative pulse is applied to grid I0. The plate-to-cathode re- -turn circuit of tube I2 includes a resistor 23, and

the voltage drop in this resistor provides the normal positive D. C. potential for the second grid 24 of a tetrode amplifier tube 25, the first grid of which is connected directly to the cathode. Thus, tubes I2 and I3 cooperate to control the on and off D I'C. voltage applied to the grid 24 of tube 25. At the same time the foregoing action is taking place, a positive impulse is being applied to Y grid I4 and a negative pulseto grid I5. At a result, the plate current of tube I1 decreases and the drop in resistance 26 decreases, causing the `plate potential of tube l5 to rise to striking level whereby current flows through resistor 21. This applies a positive D. C. potential to the second grid 28 of the tetrode amplifier tube 29. In other Words, the 0.9 k. c. control signal from source 6 causes the grids 24 and 28 of tubes 25 and 29 to `be alternately and recurrently keyed on and "off in synchronism with the half waves from source 6. y

However, the plate conductivity of each of the tubes 25 and 29 is also dependent upon the application of plate potential thereto. For this purpose, the plate potential for tube 25 is apapplied to the input terminals 33', 34 of channel 4, it is rectified in the full-Wave rectifier 35. and this rectified voltage flows through resistor 36 in the grid circuit of the triode 31 causing the grid 38 thereof to benegatively biassed to plate current cutoff. Under thiscondition, no voltage is developed across the load resistor 39. Con-V sequently, no bias is applied to the control grid 48of tube 3E). Under this condition, plate cur- The resistors 32 and 33 are connected in balanced relation across the control grid 41 and cathode 48 of a Vacuum-tube amplifier 4S which, for example, may be of a pentode type. Therefore, when both channels 4 and 5 are on space, the bias on control grid 41 is slightly negative and plate current iiows through tube 49 to deand-'3l are in the respective keying control channels 4 and 5. Thus, whenV a keyed tone is rent flows'through tube 30 and applies a steady i D. C. positive potential from power supply terminal 4I to the plate 42.

This application of potential causes current to now through resistor 32 but only during the half' cycle of the shiftcontrolfrequency from source B that appears on the grid 24. vDuring the space intervals of the keying channel 4 when no toney is present at the terminals 33' and 34, no rectification voccurs in tube V35, hence there is no bias drop across resistor 36. Therefore, plate current flows through tube31and produces a voltage drop across resistor 39 of sufficient magnitude to block the plate current through tubes 30 and 25, regardless of the'fact that'positivepulses of voltage may be applied to the grid 24. Summarizing .the above description, when a keyed tone signal appears at terminals 33T and 34, the current pulses'frorn controlsource 6 produce voltage drops across resistor 32 at the rate of 0.9 k. c. per second. This action occurs during the mark intervalsV of a channel 4. During the space intervals of this channel, no'voltage drops appear across resistor 32.

A similar action occurs when a keyed tone is applied to the input terminals 43, 44, of channel 5. These signals being rectified inthe full' waverectier 45,1to control the cutoff bias of tube 46, similar to tube 31, and consequently the L cuto bias of tube 3l. Therefore, a mar signal in channel 5 produces a voltage drop across resistor 33. When the channels 4 and 5 are on space i. e., no tone signals are applied to the input terminals thereof, no voltage drops appear across the respective resistors 32 and 33, and transmitter I remains at its normal assigned frequency. l

quency shift in the carrier is related to the amount of plate current flowing through tube 49. Therefore, when channel 4 is Yreceiving a mark signal, the voltage Ydrops across resistor 32, occurring at the rate of 0.9 k. c. per second, trigger the control grid 41'in a positive direction thus causing the frequency shifter 2 to shift the carrier to a higher frequency above its normal assigned frequency andthen back to its assigned frequency at a rate of 0.9 k. c. per second. When channel 4 is receiving a space Vsignal and channel 5 is receiving a mark signal, the voltage drops across resistor33 produce negative bias pulses 0n the control grid 41, thus shift-v ing the carrier frequency below its normal assigned frequency and .back to its normal assigned frequency at the shift rate of 0.9 k. c.rper second.

When both channels 4 and 5 are simultaneousternately produced across resistors 32 and 33 of opposite polarity at the rate of 0.9 k. c. per sec-k ond, causing grid 41 to swing positive and negative alternately and shifting the normal service frequency of the transmitter I to frequencies above and below the said normal frequency at the frequency shift rate of 0.9 k. c. per second.

As described in detail inrsaidfrapplication Serial No. 498,278, the frequency shifted carrier of the transmitter I may simultaneously be amplitude modulated from a suitable source 3 representing a third signal. A 'Y Y At the receiver the shifted frequency carrier is applied to an R. F. amplifier 52, and then to a frequency converter 53, Vsuch as ordinarily used in superheterodyne receiving systems. The resultant intermediate frequency signal is applied to the usual intermediate frequency amplifier 54, andthe output thereof is applied to a suitable beat detector 55, which is supplied with local oscillations beating frequency from the source 55 as is well-known in the art. Oscillator 55 may be such that two beat frequencies e. g., 4950 and 6750 C. P. S., are alternately produced in the output of device 55. YThese beat frequencies can be passed through suitable filters and applied to .respective signal producing or indicating devices.

of separating the two beat frequency channels at An inspection of this figure will f the receiver. show that A indicates that th'efchannel 4 frequency is 900 C. P. S. above the normal carrier B indicates that they channel frequency is4 900 C. P. S. below the normal carrier frequency and appears'as a 4950 C. P. S. beat. Channel 4 is thus illustrated as producing keying dots at a speed of 300 C. P. S. (750 words per minute Continental code). Each dot pulse is broken up into three segments representing the frequency shift rate of 900 C. P. S. Each segment therefore, will contain roughly three and one-half cycles of the 6750 C. P. S. channel 4 beat frequency. Likewise, channel 5 is illustrated as producing keying dots at a speed of 150 C. P. S. (375 Words per minute Continental code). l Each dot pulse is broken up into six segments representing the shift rate of 900 C. P. S. Each segment, therefore, will contain roughly two and one-half cycles of the 495D C. P. S. channel 5 beat frequency. It will shifter for shifting the carrier frequency of said transmitter in opposite senses and respectively in accordance with signals from said channels, a master control oscillator, a rst pair of grid-controlled electron tubes, a second pair of grid-controlled electron tubes, means connecting said osbe understood that the frequencies mentioned in the foregoing description serve merely for purposes of illustration. The rate of shift frequency, the frequency range or limits of the service carrier, the beat frequencies, and the keying speeds can be changed to suit any desired service conditions. If desired, the rate of shift frequency can also be shifted by other keyed pulses to multiply the number of channels. If the full tolerance range of the assigned service frequency of the transmitter l is utilized, it is possible to key as many as live channels or more simultaneously from the same transmitter without the usual loss of power and Without the phase distortion and troublesome side bands encountered in modulated tone multiplex methods.

Various changes and modifications may be made without departing from the spirit and scope of the invention.

What I claim is:

l. In combination, a radio service transmitter, a plurality of signalling channels, `a frequency shifter for shifting the carrier frequency of said transmitter in opposite senses and respectively in accordance with signals from said channels, a pair of grid-controlled tubes one for each channel for effectively associating each channel with the frequency shifter, a control oscillator, means connecting said oscillator in common to said pair of tubes whereby said tubes are alternately and recurrently conditioned for plate current conduction regardless of the signal condition in each channel, and means connecting each tube of said pair to its associated channel so that its conductivity is completed under control of the signal condition in the associated channel.

2. In combination a radio service transmitter, a plurality of signalling channels, a frequency shifter for shifting the carrier frequency of said transmitter in opposite senses and respectively in accordance with signals from said channels, a grid-controlled tube for controlling said shifter, means to bias the control grid of said tube in one sense to vary the frequency shifter in a like sense, means to bias said control grid in an opposite sense to vary the frequency shifter in the same sense, each of said bias means including a gridcontrolled electron tube associated with one of said channels and having means for exciting a grid thereof alternately and recurrently with a potential tending to render the tube plate conductive, and means controlled by a respective signal channel for applying a D. C. potential to the plate of the associated tube to complete the plate conductivity thereof.

3. In combination a radio service transmitter, a plurality of signalling channels, a frequency cillator to said pairs of tubes whereby one pair responds to positive half waves from the oscillator and the other pair responds to negative half waves from the oscillator, a third pair of gridcontrolled tubes having their plate current control grids connected in opposed phase relation to the cathodes of corresponding tubes in each of said first and second pairs so that the plate currents of said third pair of Vtubes are keyed alternately on and o means to apply a D. C. plate potential to one tube of said third pair under control of the first signal channel, and means .to apply a D` C. plate potential'to the other tube in said third pair under control of said second signal channel whereby each tube of said third pair becomes plate conductive only on the coincident application of said D. C. plate potential and th'e application of an on keying potential applied to its plate current control grid, and means to control said frequency shifter by the respective plate currents 0f each of said third pair of tubes and thereby controlling the operating frequency of said transmitter.

4. The combination according to claim 3 in which said first pair of grid-controlled tubes comprises a vacuum tube and a gas tube of the 'Ihyratron type, the control grids of which are connected in opposite phase to said oscillator, and said second pair of grid-controlled tubes also comprises a vacuum tube and a gas tube of the Thyratron type the control grids of which are connected in opposite phase to said oscillator, the control grid of the vacuum tube in the first pair being excited in like phase with the control grid of the gas tube in the second pair, and the control grid of the gas tube in the first pair being excited in like phase with the control grid of the vacuum tube in the second pair.

5. In combination a master control oscillator, a pair ofgrid-controlled gas tubes of the Thyratron type, means for exciting the control grids of said tubes in opposite phase from said oscillator, a rst resistor in the cathode load circuit of the first tube, a second resistor in the cathode load circuit of the second tube, a first grid-controlled vacuum tube, a second grid-controlledvacuum tube each vacuum tube having a plate current control grid, means connecting the first vacuum tube to the cathode of th'e first gas tube whereby the voltage developed in said first load resistor is applied to the said control grid of the first vacuum tube, means connecting the said control grid of the second vacuum tube to the cathode of said second gas tube whereb-y the voltage developed in said second load resistor` is applied to said control grid of said second vacuum tube, a rst signalling channel, a second signalling channel, means controlled by the signal condition in said first channel for applying a D. C. operating plate potential to the plate of said first Vacuum tube, means responsive to the signal condition in the second channel for applying a D. C. operating plate potential to the plate of th'e said second vacuum tube, a radio transmitter having a normal assigned carrier frequency, a frequency shifter for shifting the carrier frequency of said transmitter, and means for controlling the said shifter in accordance with the relative plate conductivities of said vacuum tubes.

6. The combination according Vto claim 5 in which said pair of vacuum tubes are connected across the input electrodes of another grid-con-l trolled Vvacuum tube the plate current of which controls said frequency shifter.

7. The combination according to claim 5 in which said pair of vacuum tubes each has a cathode load resistor and the resistors are connected in opposed phase relation across the input electrodesf of` another grid-controlled vacuum tube the plate current of which controls said frequency shifter. 'Y

V8. The combinationV according to claim 5 in which said transmitter is also connected to an amplitude vmodulator whereby the frequency shifted carrier is amplitude modulated under control of a third signal channel.

9. In combination a pair of signalling channels each including a tone signal source, .rectiners forrthe tone signals, and a control grid eleotron tube Whose plate current flows only when a rectified signal is present; a pair of Vgrid-conv vWhen a` rectified signalis present in the associ ated channel and its control grid is keyed on, another grid-controlled tube having its input Velectrodes connected in opposite phase relation to said pair of tubes whereby the plate current radio Vtransmitter Whose carrier frequency Yis shifted under control of said frequency shifter.

10. In combination a source of master control oscillations, a transformer having a pair Ofsec-v ondaries, means connected to one secondary to convert the positive half Waves of said source into square-topped pulses, means connected tothe other secondary to convert the 'negative half Waves from said source into square-topped pulses each of said means including a grid-controlled gas tube of the Thyratron type and a grid-controlled vacuum tube with a common plate load resistor for controlling the on and off plate striking voltage` of the gas tube, another pair of grid-controlled vacuum tubes having their control grids connected in opposite phase relation to the load circuitsrof said gas tubes, a third gridcontrolled tube having its plate current control electrodes Vconnected in opposite phase relation to the load circuits of said second pair of vacuum tubes, a frequency shifter connected to the plate circuit of said third tube, a radio transmitter` connected to said frequency shifter so as to have its operating frequency shifted thereby, a rst signalling channel for controlling the D. C. plate potential of one tube of said second pair of tubes, and another signalling channel for controlling the D. C. plate potential of the other tube of said second pair of tubes.

PHILIP BERNSTEIN. 

